The invention relates to lasers, and more particularly to lasers utilizing uniaxial crystalline quartz windows secured directly to a metal component in a hard seal.
The use of crystalline quartz or other transmissive crystalline materials such as sapphire as end members for laser tubes is known. See, for example, Wright et al. U.S. Pat. No. 4,063,803, assigned to the same assignee as the present invention. Crystalline quartz has been found to be advantageous over isotropic material such as fused silica because of its superior transmissivity and low absorption. With fused silica, ultraviolet emissions within the laser tube have tended to darken the window, producing an absorption which results in thermal heating of the window. Therefore, not only was power lost through absorption, but further degradation of the beam would occur from undesirable changes in the window's optical properties due to thermal gradients from the heating of the window.
The use of crystalline quartz as a window material, however, has carried certain disadvantages. Single-crystal quartz is not isotropic in all directions. In particular, its thermal expansion properties are not the same in all directions within the material. Single-crystal quartz has a specific atomic arrangement defined by three unique axes generally identified as X, Y and Z or A, B and C axes, all at right angles to each other. In single-crystal quartz, directional properties such as thermal expansion are the same along two of the three axes, namely X and Y (or A and B). However, directional properties are quite different in the Z (or C) direction. Single-crystal quartz thus has only one unusual axis and is said to be "uniaxial".
Materials to be hard sealed together, as by glass frit bonding, in a laser window assembly, must generally have closely matched thermal expansion coeffieients, since considerable change in temperature is experienced at the window assembly. Differences of about 10.sup.-7 .sup..degree. c.sup.-1 in thermal expansion coefficients between two materials at fritting temperature are generally regarded as undesirable, especially when one or both materials are brittle and prone to fracture.
When joints between highly mismatched materials are unavoidable, one member of the pair has conventionally been made structurally weak to accommodate the differential expansion, such as by using thin wall tubing. The weak member is reversibly or permanently deformed as the joint is cooled, without producing stress levels that could damage the joint or the mating part. In other words, the weak member is "forced" to expand and contract by deformation at the rate set by its stronger partner, and such a joint is sometime called a "forced" or "compliant" seal. Such a seal has conventionally been a solution used in crystalline quartz window assemblies.
There are certain considerations regarding reflection losses and birefringence at laser windows that have dictated that the unusual or Z axis lie in the plane of the window. Therefore the X and Z axes, or Y and Z axes, have been placed in the plane of the window to minimize losses, in accordance with conventional thinking. The reasons for this conventional preference to have the Z axis in the plane of the window will be further discussed below.
As a consequence of this conventional type of crystalline quartz window mounting, the window when heated has tended to expand unevenly, i.e. a different degree of expansion along the Z axis as compared to the X axis, with both being in the plane of the window. Therefore, the window has frequently been secured, as by glass frit bonding, to a tubular endbell component member of crystalline quartz, as an alternative to the compliant seal discussed above. In such a seal, the crystalline axes of the endbell component are in the same orientation as that of the crystal window. The window and attached end component therefore tended to expand and contract together.
The crystalline quartz tubular endbell component had to be secured in a sealed connection to another endbell component, leading ultimately to the ceramic laser plasma tube. This was usually done with a hard seal, which could be accomplished with glass frits, brazes or deformable metal gaskets. The latter can be used successfully, but require some form of clamp to compress the gasket, resulting in a rather large and cumbersome seal assembly. Therefore, a crystalline quartz tubular endbell member was often sealed in a compliant seal connection, by frit bonding, to an adjacent tubular component of somewhat malleable material, such as thin walled silver. Such connections were not mechanically strong.
It is a principal object of the present invention to form an improved laser window structure involving a crystalline quartz window with radically different crystalline orientation, enabling a simpler joint to be made to a tubular end component of matched-expansion metal.